As of 2017, phone number verification (or a paid membership) is now required to send messages. This change was made because there were several people who were attempting to create multiple accounts to send lots of messages for free.
While Oh My Japan is happy to offer a free membership that allows messages to be sent, we do need to sell a premium membership to pay for the costs of running this site. As a result, we enforce a "one person, one account" rule.
Requiring free members to register their mobile phone numbers via text message is one way to do this.